System and method for orthogonal spread spectrum sequence generation in variable data rate systems

ABSTRACT

A method and system for allocating a set of orthogonal PN code sequences of variable length among user channels operative at different data rates in a spread spectrum communication system is disclosed herein. PN code sequences are constructed that provide orthogonality between users so that mutual interference will be reduced, thereby allowing higher capacity and better link performance. In an exemplary embodiment, signals are communicated between a cell-site and mobile units using direct sequence spread spectrum communication signals. Information signals communicated on the cell-to-mobile link channels are encoded, interleaved, and modulated with orthogonal covering of each information symbol. Orthogonal Walsh function codes of varying length are employed to modulate the information signals. Code assignments are made on the basis of channel data rates in a manner which results in improved utilization of the available frequency spectrum. A substantially similar modulation scheme may be employed on the mobile-to-cell link.

This is a Continuation of application Ser. No. 08/094,822, filed Jul.20, 1993 now abandoned.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates generally to mobile communication networkssuch as, for example, cellular telephone systems. More specifically, thepresent invention relates to a novel and improved system and method forcommunicating information, in mobile cellular or satellite mobiletelephone systems, using spread spectrum communication signals.

II. Description of the Related Art

The use of code division multiple access (CDMA) modulation techniques isone of several methods for facilitating communications in systemsaccommodating a large number of users. Other multiple accesscommunication system techniques, such as time division multiple access(TDMA), frequency division multiple access (FDMA) and AM modulationschemes such as amplitude companded single sideband are known in theart. However, the spread spectrum modulation technique of CDMA hassignificant advantages over these modulation techniques for multipleaccess communication systems. The use of CDMA techniques in a multipleaccess communication system is disclosed in U.S. Pat. No. 4,901,307,issued Feb. 13, 1990, entitled "SPREAD SPECTRUM MULTIPLE ACCESSCOMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", assignedto the assignee of the present invention.

In the above-referenced U.S. Pat. No. 4,901,307, a multiple accesstechnique is disclosed where a large number of mobile telephone systemusers each having a transceiver communicate through satellite repeatersor terrestrial base stations using CDMA spread spectrum communicationsignals. In using CDMA communications, the frequency spectrum can bereused multiple times thus permitting an increase in system usercapacity. The use of CDMA results in a much higher spectral efficiencythan can be achieved using other multiple access techniques.

The CDMA techniques as disclosed in U.S. Pat. No. 4,901,307 contemplatedthe use of relatively long high speed pseudonoise (PN) sequences witheach user channel being assigned a different PN sequence. Thecross-correlation between different PN sequences and the autocorrelationof a PN sequence for all time shifts other than zero both have a zeroaverage value which allows the different user signals to bediscriminated upon reception.

However, because such PN signals are not orthogonal mutual interferencenoise is created therebetween. This interference noise arises despitethe fact that the cross-correlations of the PN signals average to zero,since for a short time interval such as an information bit time thecross-correlation follows a binomial distribution. As such, the usersignals interfere with each other much the same as if they were widebandwidth Gaussian noise at the same power spectral density.Accordingly, mutual interference noise arising from non-orthogonal usersignals tends to limit achievable system capacity.

In U.S. Pat. No. 5,103,459, issued 1992, entitled "SYSTEM AND METHOD FORGENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", alsoassigned to the assignee of the present invention, and which is hereinincorporated by reference, there is disclosed a novel and improvedmethod and system for constructing PN sequences that provideorthogonality between the users so that mutual interference will bereduced. Such a reduction in mutual interference allows for highersystem capacity and better link performance. Since with orthogonal PNcodes the cross-correlation is zero over a predetermined time interval,no mutual interference arises provided that the code time frames arealigned with each other.

In the system described in the just mentioned patent, a preferredwaveform design implemented involves a direct sequence PN spreadspectrum carrier. The chip rate of the PN carrier was chosen to be1.2288 MHz in the preferred embodiment. One consideration involved inthe choice of chip rate is that it be exactly divisible by the basebanddata rates to be used in the system. It is also desirable for thedivisor to be a power of two. In the preferred embodiment, the basebanddata rate is 9600 bits per second, leading to a choice of 1.2288 MHz,128 times 9600 for the PN chip rate.

In communications between a cellular base station and a mobile unit overthe cell-to-mobile link, the code sequences used for spreading thespectrum are constructed from two different types of sequences, eachwith different properties to provide different functions. There is anouter code that is shared by all signals in a cell or sector that isused to discriminate between multipath signals. The outer code is alsoused to discriminate between signals transmitted by different cells orsectors to the mobile units. There is also an inner code that is used todiscriminate between user signals transmitted within a single sector orcell.

In certain instances it may be desired that the voice channels within acell operate at variable data rates. The intent in using a variable datarate is to lower the data rate when there is no voice activity therebyreducing interference generated by the particular voice channel to otherusers. In this regard U.S. Patent No. 5,414,796, issued May 9, 1995,entitled "VARIABLE RATE VOCODER", also assigned to the assignee of thepresent invention, discloses a vocoder producing data at four differentdata rates based on voice activity on a 20 msec frame basis. Exemplarydata rates are 9.6 kbps, 4.8 kbps, 2.4 kbps and 1.2 kbps.

In addition to providing variable rate voice channels, it is alsodesired to allow various types of user channels (e.g., voice, facsimile,or high-speed data) to operate at different data rates. In such variabledata rate systems it may be known in advance that the maximum data rateover a certain type of user channel will never be required to exceedsome rate, e.g., 4.8 kbps, lower than a nominal system rate of 9.6 kbps.In principle, twice as many user channels should be capable of beingprovided at a data rate of 4.8 kbps relative to the number which couldbe made available at a rate of 9.6 kbps. However, if the set oforthogonal user codes were initially generated assuming a fixedtransmission rate of 9.6 kbps, the available number of codes may beinsufficient to support such a two-fold increase in the number of users.

Such a variable data rate system could also include user channels, suchas those dedicated to high-speed data transmission, requiring data ratesof double the nominal rate of 9.6 kbps, i.e., rates of 19.2 kbps. Oneobvious method for providing operation at 19.2 kbps would simply be toassign two Walsh channel codes to such users and divide the transmittedbits between the two channels. However, this would complicate systemdesign by requiring that demodulators be provided for each of the twoassigned Walsh codes.

It is therefore an object of the present invention to provide a Walshcoding technique enabling the orthogonal coexistence of high and lowdata rate channels in a spread spectrum communication system.

It is a further object of the invention that such a coding techniquemaximize the number of available channels by efficiently assigning codesof varying length to high and low data rate channels.

SUMMARY OF THE INVENTION

The implementation of CDMA techniques in the mobile cellular telephoneenvironment using orthogonal PN code sequences reduces mutualinterference between users, thereby allowing higher capacity and betterlink performance. The present invention is a novel and improved methodand system for allocating a set of orthogonal PN code sequences ofvariable length among user channels operative at different data rates.

In an exemplary embodiment, signals are communicated between a cell-siteand mobile units using direct sequence spread spectrum communicationsignals. Information signals communicated on the cell-to-mobile linkchannels are encoded, interleaved, and modulated with orthogonalcovering of each information symbol. Orthogonal Walsh function codes ofvarying length are employed to modulate the information signals. Codeassignments are made on the basis of channel data rates in a mannerwhich results in improved utilization of the available frequencyspectrum. A substantially similar modulation scheme will generally beemployed on the mobile-to-cell link.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional objects and features of the invention will be more readilyapparent from the following detailed description and appended claimswhen taken in conjunction with the drawings, in which:

FIG. 1 is a schematic overview of an exemplary CDMA cellular telephonesystem;

FIG. 2 provides an illustrative representation of a tree of Walshsequences from which a set of orthogonal Walsh codes of varying lengthmay be derived;

FIG. 3 is a block diagram of the cell-site equipment as implemented inthe CDMA cellular telephone system;

FIG. 4 is a simplified block diagram of a signal transmission subsystemto which reference will be made in describing generation of the mobileunit transmitted signals;

FIG. 5 is a block diagram of the transmission circuitry for twoexemplary voice channels;

FIG. 6 provides a block diagrammatic representation of a Walsh functiongenerator; and

FIG. 7 provides a block diagrammatic representation of an exemplary pairof receivers included within a mobile unit CDMA telephone set.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

I. System Overview

In a CDMA cellular telephone system such as is described in theabove-referenced U.S. Pat. No. 5,103,459, each cell-site has a pluralityof modulator-demodulator units or spread spectrum modems. Each modemconsists of a digital spread spectrum transmit modulator, at least onedigital spread spectrum data receiver and a searcher receiver. Eachmodem at the cell-site is assigned to a mobile unit as needed tofacilitate communications with the assigned mobile unit.

An exemplary cellular telephone system in which the present invention isembodied is illustrated in FIG. 1. The system illustrated in FIG. 1utilizes spread spectrum modulation techniques in communication betweenthe system mobile units or mobile telephones, and the cell-sites.Cellular systems in large cities may have hundreds of cell-site stationsserving hundreds of thousands of mobile telephones. The use of spreadspectrum techniques, in particular CDMA, readily facilitates increasesin user capacity in systems of this size as compared to conventional FMmodulation cellular systems.

In FIG. 1, system controller and switch 10, also referred to as mobiletelephone switching office (MTSO), typically includes interface andprocessing circuitry for providing system control to the cell-sites.Controller 10 also controls the routing of telephone calls from thepublic switched telephone network (PSTN) to the appropriate cell-sitefor transmission to the appropriate mobile unit. Controller 10 alsocontrols the routing of calls from the mobile units, via at least onecell-site, to the PSTN. Controller 10 may connect calls between mobileusers via the appropriate cell-sites since the mobile units do nottypically communicate directly with one another.

Controller 10 may be coupled to the cell-sites by various means such asdedicated telephone lines, optical fiber links, or microwavecommunication links. In FIG. 1 two such exemplary cell-sites 12 and 14are shown along with mobile units 16 and 18, where each of the units 16and 18 include a cellular telephone. Cell-sites 12 and 14 as discussedherein and as illustrated in the drawings are considered to service anentire cell. However, it should be understood that the cell may begeographically divided into sectors with each sector treated as adifferent coverage area. Accordingly, handoffs are generally required tobe made between sectors of a same cell, while diversity may also beachieved between sectors as is done between cells.

In FIG. 1, arrowed lines 20a-20b and 22a-22b respectively define thepossible communication links between cell-site 12 and mobile units 16and 18. Similarly, arrowed lines 24a-24b and 26a-26b respectively definethe possible communication links between cell-site 14 and mobile units16 and 18. Cell-sites 12 and 14 nominally transmit using equal power.

The cell-site service areas or cells are designed in geographic shapessuch that the mobile units will normally be closest to one cell-site,and within one cell sector should the cell be divided into sectors. Whenthe mobile unit is idle, i.e. no calls in progress, the mobile unitconstantly monitors pilot signal transmissions from each nearbycell-site, and if applicable from a single cell-site in which the cellis sectorized. As illustrated in FIG. 1, the pilot signals arerespectively transmitted to mobile unit 16 by cell-sites 12 and 14 uponoutbound or forward communication links 20a and 26a. Mobile unit 16 candetermine which cell it is in by comparing signal strength in pilotsignals transmitted from cell-sites 12 and 14.

In the example illustrated in FIG. 1, mobile unit 16 may be consideredclosest to cell-site 12. When mobile unit 16 initiates a call, a controlmessage is transmitted to the nearest cell-site, cell-site 12. Cell-site12, upon receiving the call request message, transfers the called numberto system controller 10. System controller 10 then connects the callthrough the PSTN to the intended recipient.

Should a call be initiated within the PSTN, controller 10 transmits thecall information to all the cell-sites in the area. The cell-sites inreturn transmit a paging message within each respective coverage areathat is intended for the called recipient mobile user. When the intendedrecipient mobile unit hears the page message, it responds with a controlmessage that is transmitted to the nearest cell-site. This controlmessage signals the system controller that this particular cell-site isin communication with the mobile unit. Controller 10 then routes thecall through this cell-site to the mobile unit. Should mobile unit 16move out of the coverage area of the initial cell-site 12, an attempt ismade to continue the call by routing the call through another cell-site.

With respect to cellular telephone systems, The Federal CommunicationsCommission (FCC) has allocated a total of 25 MHz for mobile-to-celllinks and 25 MHz for cell-to-mobile links. The FCC has divided theallocation equally between two service providers, one of which is thewireline telephone company for the service area and the other chosen bylottery. Because of the order in which allocations were made, the 12.5MHz allocated to each carrier for each direction of the link is furthersubdivided into sub-bands. For the wireline carriers, the sub-bands areeach 10 MHz and 2.5 MHz wide. For the non-wireline carriers, thesub-bands are each 11 MHz and 1.5 MHz wide. Thus, a signal bandwidth ofless than 1.5 MHz could be fit into any of the sub-bands, while abandwidth of less than 2.5 MHz could be fit into all but one sub-band.

To preserve maximum flexibility in allocating the CDMA technique to theavailable cellular frequency spectrum, the waveform utilized in thecellular telephone system should be less than 1.5 MHz in bandwidth. Agood second choice would be a bandwidth of about 2.5 MHz, allowing fullflexibility to the wireline cellular carriers and nearly fullflexibility to non-wireline cellular carriers. While using a widerbandwidth has the advantage of offering increased multipathdiscrimination, disadvantages exist in the form of higher equipmentcosts and lower flexibility in frequency assignment within the allocatedbandwidth.

In a spread spectrum cellular telephone system, such as illustrated inFIG. 1, the preferred waveform design implemented involves a directsequence pseudonoise spread spectrum carrier. The chip rate of the PNsequence is chosen to be 1.2288 MHz in the preferred embodiment. Thisparticular chip rate is chosen so that:

(i) the resulting bandwidth thereof, about 1.25 MHz after filtering, isapproximately one-tenth of the total bandwidth allocated to one cellularservice carrier; and

(ii) it may be accommodated by the smallest, i.e., the 1.5 MHz,frequency sub-band mentioned above.

Another consideration in the choice of the exact chip rate is that it isdesirable that the chip rate be exactly divisible by the baseband datarates to be used in the system. It is also desirable for the divisor tobe a power of two. In an exemplary embodiment, at least one user channeloperates at a baseband date rates of 9600 bits per second, leading to achoice of 1.2288 MHz, 128 times 9600 for the PN chip rate. Otherchannels utilize baseband data rates which are multiples orsub-multiples of the 9600 rate.

In the cell-to-mobile link, the binary sequences used for spreading thespectrum are constructed from two different types of sequences, eachwith different properties to provide different functions. There is anouter code that is shared by signals in a cell or sector that is used todiscriminate between multipath signals. The outer code is also used todiscriminate between signals transmitted by different cells or sectorsto the mobile units. There is also an inner code that is used todiscriminate between user signals transmitted by a single sector orcell. As is described hereinafter, one aspect of the present inventionis directed to an improved method for allocating such inner codes amongthe various user channels within a particular sector or cell.

The carrier waveform design in the preferred embodiment for thecell-site transmitted signals utilizes a sinusoidal carrier that isquadraphase (four phase) modulated by a pair of binary PN sequences thatprovide the outer code transmitted by a single sector or cell. As isdescribed hereinafter, modulation is also provided by an inner codeformulated in accordance with the invention. The outer code sequencesare generated by two different PN generators of the same sequencelength. Once sequence bi-phase modulates the in-phase channel (IChannel) of the carrier and the other sequence bi-phase modulates thequadrature-phase (Q Channel) of the carrier. The resulting signals aresummed to form a composite four-phase carrier.

Although the values of a logical "zero" and a logical "one" areconventionally used to represent the binary sequences, the signalvoltages used in the modulation process are +V volts for a logical "one"and -V volts for a logical "zero". To bi-phase modulate a sinusoidalsignal, a zero volt average value sinusoid is multiplied by the +V or -Vvoltage level as controlled by the binary sequences using a multipliercircuit. The resulting signal may then be band limited by passingthrough a bandpass filter. It is also known in the art to lowpass filterthe binary sequence stream prior to multiplying by the sinusoidalsignal, thereby interchanging the order of the operations. A quadraphasemodulator consists of two bi-phase modulators each driven by a differentsequence, with the sinusoidal signals used in the bi-phase modulatorshaving a 90° phase shift therebetween.

In the preferred embodiment, the sequence length for the transmittedsignal carrier is chosen to be 32,768 chips. Sequences of this lengthcan be generated by a modified maximal-length linear sequence generatorby adding a zero bit to a length 32,767 chip sequence. The resultingsequence has good cross-correlation and autocorrelation properties. Goodcross-correlation and autocorrelation properties are necessary toprevent mutual interference between pilot carriers transmitted bydifferent cells.

A sequence this short in length is desirable in order to minimizeacquisition time of the mobile units when they first enter the systemwithout knowledge of system timing. With unknown timing, the entirelength of the sequence must be searched to determine the correct timing.The longer the sequence, the longer time the acquisition search willrequire. Although sequences shorter than 32,768 could be used, it mustbe understood that as sequence length is reduced the code processinggain may be diminished. As processing gain diminishes, the rejection ofmultipath interference along with interference from adjacent cells andother sources will also be reduced, perhaps to unacceptable levels.Thus, there is a desire to use the longest sequence that can be acquiredin a reasonable time. It is also desirable to use the same codepolynomials in all cells so that the mobile unit, not knowing what cellit is in when initially acquiring synchronization, can obtain fullsynchronization by searching a single code polynomial.

In order to simplify the synchronization process, all the cells in thesystem are synchronized to each other. In the exemplary embodiment, cellsynchronization is accomplished by synchronizing all the cells to acommon time reference, the Navstar Global Positioning System satellitenavigation system which is itself synchronized to Universal CoordinatedTime (UTC).

Signals from different cells are differentiated by providing timeoffsets of the basic sequences. Each cell is assigned a different timeoffset of the basic sequences differing from its neighbors. In thepreferred embodiment, the 32,768 repetition period is divided into a setof 512 timing offsets. The 512 offsets are spaced 64 chips apart. Eachsector of each cell in a cellular system is assigned a different one ofthe offsets to use for all its transmissions. If there are more than 512sectors or cells in the system, then the offsets can be reused in thesame manner as frequencies are reused in the present analog FM cellularsystem. In other designs, a different number than 512 offsets could beused. With reasonable care in assignment of pilot offsets, it shouldnever be necessary for near neighboring cells to use near neighboringtime offsets.

All signals transmitted by an omnidirectional cell or one of the sectorsof a sectored cell share the same outer PN codes for the I and Qchannels. The signals are also spread with an inner orthogonal codegenerated by using Walsh functions. A signal addressed to a particularuser is multiplied by the outer PN sequences and by a particular Walshsequence, or sequence of Walsh sequences, assigned by the systemcontroller for the duration of the user's telephone call. The same innercode is applied to both the I and Q channels resulting in a modulationwhich is effectively bi-phase for the inner code.

It is well known in the art that a set of n orthogonal binary sequences,each of length n, for n any power of 2 can be constructed, see DigitalCommunications with Space Applications, S. W. Golomb et al.,Prentice-Hall, Inc., 1964, pp. 45-64. In fact, orthogonal binarysequence sets are also known for most lengths which are multiples offour and less than two hundred. One class of such sequences that is easyto generate is called the Walsh function, also known as Hadamardmatrices.

A Walsh function of order n can be defined recursively as follows:##EQU1## where W' denotes the logical complement of W, and W(1)=0. Thus,##EQU2## W(8) is as follows: ##EQU3## A Walsh sequence or code is one ofthe rows of a Walsh function matrix. A Walsh function of order ncontains n code sequences, each of length n bits.

A Walsh function of order n (as well as other orthogonal functions) hasthe property that over the interval of n code symbols, thecross-correlation between all the different sequences within the set iszero, provided that the sequences are time aligned with each other. Thiscan be seen by noting that every sequence differs from every othersequence in exactly half of the bits. It should also be noted that thereis always one sequence containing all zeroes and that all the othersequences contain half ones and half zeroes.

Neighboring cells and sectors can reuse the Walsh sequences because theouter PN codes used in neighboring cells and sectors are distinct.Because of the differing propagation times for signals between aparticular mobile's location and two or more different cells, it is notpossible to satisfy the condition of time alignment required for Walshfunction orthogonality for both cells at one time. Thus, reliance mustbe placed on the outer PN code to provide discrimination between signalsarriving at the mobile unit from different cells. However, all thesignals transmitted by a cell are orthogonal to each other and thus donot contribute interference to each other. This eliminates the majorityof the interference in most locations, allowing a higher capacity to beobtained.

The system further envisions the voice channel to be a variable ratechannel whose data rate can be varied from data block to data block withminimum of overhead required to control the data rate in use. The use ofvariable data rates reduces mutual interference in neighboring cells,and saves cell power, by eliminating unnecessary transmissions whenthere is no useful speech to be transmitted. Algorithms are utilizedwithin the vocoders for generating a varying number of bits in eachvocoder block in accordance with variations in speech activity. Duringactive speech, the vocoder may produce 20 msec. data blocks containing,20, 40, 80, or 160 bits, depending on the voice activity of the speaker.It is desired to transmit the data blocks in a fixed amount of time byvarying the rate of transmission. It is further desirable not to requiresignalling bits to inform the receiver how many bits are beingtransmitted.

Although the present invention is described hereinafter in the contextof the cell-to-mobile link within the communication system of FIG. 1, itis understood that the mobile-to-cell link may be similarly implementedin accordance with the teachings herein. In the preferred embodiment,orthogonal Walsh functions of varying length are assigned to userchannels on the cell-to-mobile link. In particular, each channel isassigned a unique orthogonal Walsh sequence having a length predicatedon the channel data rate. In the case of voice channels, the digitalsymbol stream for each voice signal is multiplied by its assigned Walshsequence. The Walsh coded symbol stream for each voice channel is thenmultiplied by the outer PN coded waveform. The resultant spread symbolstreams are then added together to form a composite waveform.

The resulting composite waveform is then modulated onto a sinusoidalcarrier, bandpass filtered, translated to the desired operatingfrequency, amplified and radiated by the antenna system. Alternateembodiments of the present invention may interchange the order of someof the just described operations for forming the cell-site transmittedsignal. For example, it may be preferred to multiply each voice channelby the outer PN coded waveform and perform the filter operation prior tosummation of all the channel signals to be radiated by the antenna. Itis well known in the art that the order of linear operations may beinterchanged to obtain various implementation advantages and differentdesigns.

The waveform design of the preferred embodiment for cellular serviceuses the pilot carrier approach for the cell-to-mobile link, as isdescribed in U.S. Pat. No. 4,901,307. All cells transmit pilot carriersusing the same 32,768 length sequence, but with different timing offsetsto prevent mutual interference.

As is described in greater detail below, the symbol stream for aparticular cellular user is combined in a first exclusive OR operationwith the Walsh sequence assigned to the user. The Walsh function isclocked at a rate of 1.2288 MHz, while in an exemplary variable datarate system including voice, facsimile (FAX), and high/low-speed datachannels the information symbol rate may vary from approximately 75 Hzto 76,800 Hz. The resultant coded waveform is combined in a secondexclusive OR operation with a binary PN sequence also clocked at 1.2288MHz. An identical binary PN sequence is used to encode each mobilechannel within a particular sector of the coverage area of the cellularsystem. As a consequence of the orthogonality of the Walsh codingsequences, each may be used on a single RF channel associated with sucha sector without inducing interference among the users within thesector.

The signals carried by each channel may further be convolutionallyencoded, with repetition, and interleaved in order to provide errordetection and correction functions which allow the system to operate ata much lower signal-to-noise and interference ratio. Techniques forconvolutional encoding, repetition and interleaving are well known inthe art. The resulting signals are then generally modulated onto an RFcarrier and summed with the pilot and setup carriers, along with theother voice carriers. Summation may be accomplished at several differentpoints in the processing such as at the IF frequency, or at the basebandfrequency either before or after multiplication by the PN sequenceassociated with the channels within a particular cell.

Each voice carrier may also be multiplied by a value that sets itstransmitted power relative to the power of the other voice carriers.This power control feature allows power to be allocated to those linksthat require higher power due to the intended recipient being in arelatively unfavoring location. Means are provided for the mobiles toreport their received signal-to-noise ratio to allow the power to be setat a level so as to provide adequate performance without waste. Theorthogonality property of the Walsh functions is not disturbed by usingdifferent power levels for the different voice carriers provided thattime alignment is maintained.

II. Variable-Length Walsh Sequence Coding

In the system described in U.S. Pat. No. 5,103,459, a maximum of 61users with three overhead channels were capable of being accommodated ina given cell sector when each channel was coded using a unique Walshsequence of length 64. In accordance with the invention, user channelsare assigned codes of varying length based on the data rate of eachchannel. This allows orthogonal coding of a larger number of userswithin a given sector by allowing more orthogonal codes to be used thanwhen codes are constrained to be of a fixed length. It should beunderstood that while in the exemplary embodiment, 64 Walsh sequencesare used, more or less channels may be formed using other orders ofWalsh functions in other systems with the teaching of the presentinvention applicable.

As may be appreciated with reference to the recursive formula forgenerating Walsh functions set forth by equation (1), Walsh functionmatrices of progressively higher order are derived from lower-orderWalsh matrices. Since a Walsh sequence corresponds to a particular rowwithin a Walsh function matrix, it is possible to form a table, or"tree", of Walsh sequences through recursive application of equation(1).

FIG. 2 provides an illustrative representation of a tree of Walshsequences in which the lowest order Walsh sequence of the tree, i.e.,the "root" sequence, is equivalent to "0". Such a tree of Walshsequences may be envisioned as a set of interconnected nodes each havingtwo branches, where all of the nodes may be traced back to the rootnode. Assuming the first order Walsh sequence used to define the rootnode is denoted as W₀ (1), then the two branches from this root nodewill be connected to a pair of nodes defined by the Walsh sequences "00"and "01", which comprise the rows of the Walsh function matrix W₀ (2).As is indicated by FIG. 2, this process may be continued by deriving aWalsh function matrix W₁ (2) by substituting W₁ (1)=00 into equation(1). This results in the node "00"branching into the nodes "0000" and"0101", which comprise the rows of W₁ (2). A similar recursivesubstitution yields branch nodes "0101" and "0110" from node "01".

It should be noted that sequences more complex than "0" can be used todefine the root node of a tree of Walsh sequences. For example, if itwere desirable for the number of chips in the mobile channel codes to bea multiple of three, then the sequence "010" could be used for the rootof the tree. The Walsh sequences associated with the remaining nodes ofthe tree would then be derived as before via recursive application ofequation (1).

Clearly, the Walsh sequence defining a node of the tree branching from agiven node is not orthogonal to the Walsh sequence associated with thegiven node. It follows that assignment to a mobile channel of a Walshsequence corresponding to a particular node would preclude assignment ofWalsh sequences associated with nodes connected, either above or belowthe given node, by a set of branches not including any "turns", i.e.,directional changes. However, any other Walsh sequences defining nodesnot connected to the given node in this manner could be simultaneouslyused as codes to identify other mobile channels.

One way of characterizing particular sets of nodes capable ofsimultaneous assignment is in terms of a "family tree". In this contextthe nodes linearly above a particular node correspond to the "ancestors"of that node, while those nodes linearly below the node may be describedas "descendants". Nodes which are lineal ancestors or descendants of agiven node are not orthogonal to the node. Conversely, all other nodesin the tree are orthogonal to the given node. For example, each of thenodes in FIG. 2 within the group G1 may be considered to belineally-related and hence ineligible for simultaneous assignment. Incontrast, neither node "0110"nor node "01101001" within group G2 islineally-related to node "0101". Accordingly, these Walsh sequence codeswould be capable of being contemporaneously allocated to channels ofdiffering data rates.

A set of Walsh sequence codes derived from the Walsh tree representationof FIG. 2 is set forth below in Table I. Included within Table I areWalsh sequence codes derived from Walsh function matrices W(1), W(2),W(4), W(8), and W(16), where, again, W(1)=0 corresponds to the root nodein FIG. 2. Each Walsh code sequence is identified by a Code Label, X/Y,wherein Y represents the length (in chips) of the code and X denotes acode number (0≦X≦Y-1). Although the maximum length of the code sequencesincluded within Table I is 16 chips, it is clear that longer sequencescould be derived through repeated recursive application of equation (1).

                  TABLE I                                                         ______________________________________                                        Code Label (X/Y)   Code                                                       ______________________________________                                        0/1                0                                                          0/2                00                                                         1/2                01                                                         0/4                0000                                                       1/4                0101                                                       2/4                0011                                                       3/4                0110                                                       0/8                00000000                                                   1/8                01010101                                                   2/8                00110011                                                   3/8                01100110                                                   4/8                00001111                                                   5/8                01011010                                                   6/8                00111100                                                   7/8                01101001                                                   0/16               0000000000000000                                           1/16               0101010101010101                                           2/16               0011001100110011                                           3/16               0110011001100110                                           4/16               0000111100001111                                           5/16               0101101001011010                                           6/16               0011110000111100                                           7/16               0110100101101001                                           8/16               0000000011111111                                           9/16               0101010110101010                                           10/16              0011001111001100                                           11/16              0110011010011001                                           12/16              0000111111110000                                           13/16              0101101010100101                                           14/16              0011110011000011                                           15/16              0110100110010110                                           ______________________________________                                    

If orthogonality is to be maintained between a set of user channelsassigned Walsh codes from Table I, then codes associated withbranch-connected nodes in the Walsh tree representation of FIG. 2 maynot be simultaneously utilized. That is, neither longer code sequencesrecursively derived from a given code in accordance with equation (1),nor shorter code sequences from which the given code was recursivelyderived, may be assigned to other communication channels when the givencode is in use. In an exemplary embodiment, Walsh sequence codes of thelongest available length (i.e., 16 chips in a system utilizing thecoding scheme of Table I) are assigned to the channels having the lowestdata rate. Illustratively, codes 0/16 through 15/16 could be assigned tolow data rate channels in which a single data symbol is transmittedduring each 16 code chips. Other system channels requiring twice thislowest data rate could then be allocated one of the codes from 0/8 to7/8.

As an example, assume code 4/8 has been assigned to a user channeltransmitting information symbols at twice the lowest data rate. As aconsequence of this assignment codes 4/16 and 12/16 could not beallocated to lower data rate channels, since these codes correspond tothe rows of the Walsh function matrix created by substituting code 4/8into equation (1). Similarly, codes 0/2 and 0/4 could not be assigned tohigher data rate channels since code 4/8 could be derived therefrom.

As an additional example, assume code 1/4 is assigned to a channeloperating at four times the lowest data rate. This assignment wouldpreclude the allocation of channels 1/8, 5/8, 1/16, 5/16, 9/16, and13/16 to lower data rate channels and code 1/2 to higher data ratechannels. It is noted that the use of code 0/1 prohibits the assignmentof any other code because each of the remaining codes could be derivedfrom code 0/1.

III. Channel Assignment of Variable-Length Walsh Codes

In the preferred embodiment, a cell controller monitors existing Walshcode assignments in order to enable additional requests for channelscodes to be efficiently accommodated. This monitoring could be effectedby, for example, annotating a tabular list of the Walsh code sequenceseach time a code is assigned to a particular channel. When it is desiredto initiate an additional code assignment, a set of potentiallyassignable codes are identified by searching the list. This set of codeswould include only those codes not recursively related by equation (1)to a currently assigned code. That is, the set would include only thosecodes not capable of being derived from an assigned code, and the codesfrom which the assigned code is capable of being derived.

Subsequent to selection of a particular code, all codes recursivelyrelated in the above-described manner to the selected code would bemarked unavailable. When communication over a particular channel isterminated, such as at the conclusion of a telephone call made over avoice channel, the unavailable designation would be removed from thecorresponding channel code. However, prior to marking the correspondingchannel code as available it would need to be determined that anypotentially conflicting codes were not currently assigned.

In a particular implementation the cell controller would maintain anASSIGNED list of the set of codes already assigned to particular userchannels, and would further include a separate "BUSY" list having anentry for each possible Walsh code. When a request for a channel code ismade, the controller would first reset, i.e., clear, the BUSY list. Eachof the entries in the BUSY list corresponding to codes currentlyincluded in the ASSIGNED list would then be marked as being busy. Inaddition, all entries within the BUSY list corresponding to codesrecursively related to those indicated as being busy would also bemarked as being busy. Next, the BUSY list would be searched for anavailable code having a chip length appropriate for the data rate of therequesting channel. This information could be by provided by, forexample, a LENGTH table within the cell controller specifying the codessuitable for allocation to channels operative at various data rates.Upon identification of a code of suitable length, the controller wouldassign the identified code to the requesting channel. At the conclusionof channel communication the assigned code is deleted from the ASSIGNEDlist.

After extended periods of system operation the particular set of Walshcodes included in the ASSIGNED list may inefficiently limit the numberof remaining available channels. For example, random assignment of codesto low data rate channels may preclude allocation of a large number ofshorter-length codes, recursively related to the randomly assignedlonger-length codes, to high data rate channels. Accordingly, it willgenerally be advantageous to assign codes to low data rate users in sucha way as to minimize the number of disqualified shorter-length codes. Inparticular implementations this could be effected by, for example,initially assigning those longer-length codes recursively related byequation (1) to shorter-length codes included on the BUSY list ofdisqualified codes. Only after all such longer-length codes had beenassigned would codes not so recursively related be considered forallocation to low data rate users. Moreover, it is expected to beadvantageous to assign users operative at a particular data rate toclosely related codes so as to minimize the number of high data rate,i.e., shorter-length, codes marked as unavailable. The availability ofshort-length codes may be further increased by reassigning codes amongactive lower data rate channels so that all eligible longer-length codeslineally-related to a given shorter-length code are used prior toallocation of codes not lineally-related to the shorter-length code.

IV. Apparatus for Variable-Length Walsh Coding in a Cellular System

FIG. 3 illustrates in block diagram form an exemplary embodiment ofcell-site equipment. At the cell-site, two receiver systems are utilizedwith each having a separate antenna and analog receiver for spacediversity reception. In each of the receiver systems the signals areprocessed identically until the signals undergo a diversity combinationprocess. The elements within the dashed lines correspond to elementscorresponding to the communications between the cell-site and one mobileunit. The output of the analog receivers are also provided to otherelements used in communications with other mobile units.

In FIG. 3, the first receiver system is comprised of antenna 30, analogreceiver 32, searcher receiver 34 and digital data receiver 36. Thefirst receiver system may also include an optional digital data receiver38. The second receiver system includes antenna 40, analog receiver 42,searcher receiver 44 and digital data receiver 46. Although not fullyillustrated, it is preferred that there be multiple digital datareceivers (e.g. two to four) associated with each analog receiver forcell-site circuitry (illustrated in dashed lines) assigned forcommunication with a particular mobile unit. Although not shown itshould further be understood that the data receivers may be assigned toprocess the signal from either or both of the analog receivers.

The cell-site also includes cell-site control processor 48. Controlprocessor 48 is coupled to data receivers 36, 38, and 46 along withsearcher receivers 34 and 44. Control processor 48 provides among otherfunctions, functions such as signal processing; timing signalgeneration; power control; and control over handoff, diversity,combining and system control processor interface with the MTSO. Walshsequence assignment along with transmitter and receiver assignment isalso provided by control processor 48. In a preferred embodiment, thecontrol processor 48 will include a table of orthogonal Walsh codesequences, generated recursively as described above, of chip lengthsranging from 2 to 1024. This range of sequence length enables orthogonalWalsh codes to be provided for data rates with rate one-halfconvolutional coding of between 600 bps and 307200 bps, it beingunderstood that Walsh codes of longer length could be generated tofacilitate transmission at even lower data rates. After a Walsh sequencehas been selected using the BUSY and ASSIGNED tables, it is assigned bycontrol processor 48 at call setup time in the same manner as channelsare assigned to calls in the analog FM cellular system.

Both receiver systems are coupled by data receivers 36, 38, and 46 todiversity combiner and decoder circuitry 50. Digital link 52 is coupledto receive the output of diversity combiner and decoder circuitry 50.Digital link 52 is also coupled to control processor 48, cell-sitetransmit modulator 54 and the MTSO digital switch. Digital link 52 isutilized to communicate signals to and from the MTSO with cell-sitetransmit modulator 54 and circuitry 50 diversity combiner and decoderunder the control of control processor 48.

FIG. 4 is a simplified block diagram of a signal transmission subsystem150 to which reference will be made in describing generation of themobile unit transmitted signals. As shown in FIG. 4, baseband datacorresponding to channels 1, 2, . . . N, is provided to a plurality ofmultipliers 154. The baseband data signals are modulated by a set oforthogonal PN sequences provided by orthogonal PN sequence generators160. Each of the PN signals is synthesized on the basis of an address ofa particular mobile unit and is clocked at a predetermined rate, whichin the preferred embodiment is 1.2288 MHz. This clock rate is chosen tobe an integer multiple of one of the baseband data rates, e.g., 9.6kbps. As is described in detail below, data from each voice channel isalso encoded with repetition, interleaved, scrambled, and multiplied byits assigned Walsh code sequence.

Referring again to FIG. 4, the set of orthogonal PN spread data signalsproduced by multipliers 154 are supplied to a summation network 168.Within the summation network 168 this set of orthogonal data signals isfurther spread by a pilot PN sequence associated with a particularcellular region. The spread spectrum data generated by summation network168 is utilized by an RF transmitter in modulating an RF carrierprovided to an antenna 180. The RF transmission from the antenna 180 isused to communicate the set of direct sequence spread spectrum signalsproduced by subsystem 150 to the various mobile units within theassociated cellular region.

In the exemplary embodiment of the present invention, the voice channelutilizes a variable data rate. The intent in using a variable data rateis to lower the data rate when there is no voice activity therebyreducing interference generated by this particular voice channel toother users. The vocoder envisioned to provide variable rate data isdisclosed in the above-referenced copending U.S. patent applicationentitled "VARIABLE RATE VOCODER". Again, such a vocoder produces data atfour different data rates based on voice activity on a 20 msec framebasis (e.g., 9.6 kbps, 4.8 kbps, 2.4 kbps and 1.2 kbps). Although thedata rate will vary on a 20 msec basis, the code symbol rate is keptconstant by code repetition at 19.2 ksps. It is noted that utilizationof 19.2 ksps generally presupposes a nominal maximum data rate of 9.6kbps. Accordingly, at an information bit rate of 9.6 kbps there is norepetition of the convolutional code symbols produced by the vocoder. Ata bit rate of 4.8 kbps each code symbol is repeated twice, while at bitrates of 2.4 kbps and 1.2 kbps the code symbols are repeated four timesand eight times, respectively.

Since the variable rate scheme is devised to reduce interference, thecode symbols at the lower rates will have lower energy. For example, forthe exemplary data rates of 9.6 kbps, 4.8 kbps, 2.4 kbps and 1.2 kbps,the code symbol energy (E_(s)) is respectively E_(b) /2, E_(b) /4, E_(b)/8 and E_(b) /16 where E_(b) is the information bit energy for the 9.6kbps transmission rate.

The code symbols are interleaved by a convolutional interleaver suchthat code symbols with different energy levels will be scrambled by theoperation of the interleaver. In order to keep track of what energylevel a code symbol should have a label is attached to each symbolspecifying its data rate for scaling purposes. After orthogonal Walshcovering and PN spreading, the quadrature channels are digitallyfiltered by a Finite Impulse Response (FIR) filter. The FIR filter willreceive a signal corresponding to the symbol energy level in order toaccomplish energy scaling according to the data rate. The I and Qchannels will be scaled by factors of: 1, 1/√2, 1/2, or 1/2√2. In oneimplementation the vocoder would provide a data rate label in the formof a 2-bit number to the FIR filter for controlling the filter scalingcoefficient.

In FIG. 5, the circuitry of two exemplary voice channels, voice channels(i) and (j) are illustrated. The voice channel (i) data is input from anassociated vocoder (not shown) to transmit modulator 54 (FIG. 3).Transmit modulator 54 is comprised of encoder 250_(i), interleaver251_(i), exclusive-OR gates 252_(i), 255_(i), 256_(i), and 258_(i), PNgenerator 253_(i), and Walsh generator (W_(i)) 254_(i). Data Rate Selectsignals, indicative of the lengths of the Walsh codes assigned tochannels (i) and (j), are provided by control processor 48 to each ofthe elements within the modulator 54.

The voice channel (i) data is input to encoder 250i where in theexemplary embodiment it is convolutionally encoded with code symbolrepetition in accordance with the input data rate and sequence length ofthe assigned Walsh code. The encoded data is then provided tointerleaver 251i where, in the exemplary embodiment, it isconvolutionally interleaved. Interleaver 251i also receives from thevocoder associated with the voice channel (i) a 2-bit data rate labelthat is interleaved with the symbol data to identify the data rate tothe FIR filters. The data rate label is not transmitted. At the mobileunit, the decoder checks for all possible codes. The interleaved symboldata is output from interleaver 251_(i) at an exemplary rate of 19.2ksps to an input of exclusive-OR gate 251_(i).

In the exemplary embodiment, each voice channel signal is scrambled toprovide greater security in cell-to-signal mobile transmissions.Although such scrambling is not required it does enhance the security incommunications. For example, scrambling of the voice channel signals maybe accomplished by PN coding the voice channel signals with a PN codedetermined by the mobile unit address of user ID. Such PN scrambling maybe provided by the PN generators 253_(i) and 253_(j) (FIG. 5) using asuitable PN sequence or encryption scheme. Although in the preferredembodiment, scrambling is performed using a PN sequence, scrambling maybe accomplished by other techniques including those well known in theart.

Referring again to FIG. 5, scrambling of the voice channel (i) signalmay be accomplished by providing PN generator 253_(i) which receives theassigned mobile unit address from the control processor. PN generator253_(i) generates a unique PN code that is provided as the other inputto exclusive-OR gate 251_(i). The output of exclusive-OR gate 251_(i) isthen provided to one input of exclusive-OR gate 255_(i).

Walsh generator (W_(i)) 254_(i) generates, in response to a functionselect signal and timing signals from the control processor, a signalcorresponding to the Walsh sequence assigned to channel (i). The valueof the function select signal may be determined by the address of themobile unit. The Walsh sequence signal is provided as the other input toexclusive-OR gate 255_(i). The scrambled symbol data and Walsh sequenceare exclusive-OR'ed by exclusive-OR gate 255_(i) with the resultprovided as an input to both of exclusive-OR gates 256_(i) and 258_(i).PN generator 253_(i) along with all other PN generators and Walshgenerators at the cell-site provide an output at 1.2288 MHz. It shouldbe noted that PN generator 253_(i) includes a decimator which providesan output at a 19.2 kHz rate to exclusive-OR gate 252_(i).

The other input of exclusive-OR gate 256_(i) receives a PN_(I) signalwhile the other input of exclusive-OR gate 258_(i) receives a PN_(Q)signal. The PN_(I) and PN_(Q) signals are pseudorandom noise sequencescorresponding to a particular cell sector or address, and relaterespectively to the In-Phase (I) and Quadrature (Q) channels. The PN_(I)and PN_(Q) signals are respectively exclusive-OR'ed with the output ofexclusive-OR gate 251_(i) and respectively provided as inputs to FiniteImpulse Response (FIR) filters 260_(i) and 262_(i). The input symbolsare filtered according to the input data rate label (not shown) fromconvolutional interleaver 251_(i). The filtered signals output from FIRfilters 260_(i) and 262_(i) are provided to a portion of transmit powercontrol circuitry comprised of gain control elements 264_(i) and266_(i). The signals provided to gain control elements 264_(i) and266_(i) are gain controlled in response to input signals (not shown)from the control processor. The signals output from gain controlelements are provided to transmit power amplifier circuitry.

FIG. 5 further illustrates voice channel (j) which is identical infunction and structure to voice channel (i). It is contemplated thatthere exist many more voice channels (not illustrated), where anarbitrarily large number of channels may be accommodated by expandingthe size of Table I.

FIG. 6 provides a block diagrammatic representation of a preferredimplementation of the Walsh generator 254_(i) shown in FIG. 5. Thegenerator 254_(i) includes an N-bit binary counter 350, to which isprovided a clock signal at a frequency equivalent to the 1.2288 MHzspread spectrum chip rate used in the exemplary embodiment. The number,N, of bit registers 354 in the binary counter 350 is determined by theratio of the log (base 2) of the spread spectrum chip rate to the lowestdesired channel symbol rate. For example, N=10 in a system operative ata chip rate of 1.2288 MHz in which the lowest data rate is 1200symbols/second.

Each bit register 354 is connected to an input of an AND gate 360. Theremaining input to each AND gate 360 is provided by one of the controlbits within a N-bit Walsh Function Select word supplied by the controlprocessor 48. As shown in FIG. 6, each of the AND gates 360 drives oneof N-1 exclusive-OR gates 364. The AND gates 360 are connected with theexclusive-OR gates 364 such that the logical parity function of the worddefined by the outputs of the AND gates 360 is produced at the output ofthe exclusive-OR gate 364'.

The Walsh generator of FIG. 6 is capable of generating Walsh functionsof length 2^(n), for n=1, 2, . . . N in accordance with the 10-bit Walshfunction select signal. In the exemplary embodiment, the Walsh generator254_(j) will be substantially identical to the generator 254_(i).

In the exemplary embodiment for the cell-to-mobile link, the voicechannels use convolutional encoding of a constraint length K=9 and coderate r=1/2, that is, two encoded symbols are produced and transmittedfor every information bit to be transmitted. In addition to theconvolutional encoding, convolutional interleaving of symbol data isalso employed. It is further envisioned that repetition is utilized inconjunction with the convolutional encoding. At the mobile unit, theoptimum decoder for this type of code is the soft decision Viterbialgorithm decoder. A standard design can be used for decoding purposes.The resulting decoded information bits are passed to the mobile unitdigital baseband equipment.

Referring again to FIG. 5, the transmitter circuitry includes a seriesof digital to analog (D/A) converters for converting the digitalinformation from the PN_(I) and PN_(Q) spread data for the voicechannels to analog form. In particular, the voice channel PN_(I) spreaddata is output from gain control elements 264_(i) -264_(j) to D/Aconverters 280_(i) -280_(j), respectively. Similarly, the output of thecorresponding gain control elements for PN_(Q) spread data, i.e. gaincontrol elements 264_(i) -264_(j), are respectively provided to D/Aconverters 282_(i) -282_(j). The digitized data from the D/A convertersis mixed with an RF signal so as to provide frequency upconversion. Inaccordance with well-known techniques, the resulting RF signal willtypically be bandpass filtered and amplified prior to transmission.

Cell-site control processor 48 (FIG. 3) has the responsibility forassignment of digital data receivers and transmit modulators to aparticular call. Control processor 48 also monitors the progress of thecall, quality of the signals, and initiates teardown on loss of signal.The cell-site communicates with the MTSO via link 52 where it is coupledby a standard telephone wire, optical fiber, or microwave link.

The control processor 48 also has responsibility for assignment ofdigital data receivers and modulators at the cell-site to handleparticular calls. Thus, in the cell-to-mobile link, the controlprocessor controls the assignment of Walsh sequences used at thecell-site in transmission of a particular call to the mobile unit. Inaddition, the system control processor controls the receiver Walshsequences and PN codes. In the mobile-to-cell link, the controlprocessor within the mobile unit also controls the mobile unit user PNcodes for the call. Assignment information is therefore transmitted fromthe MTSO to the cell-site and from there to the mobile. The controlprocessor within the mobile unit also monitors the progress of the call,the quality of signals, and initiates tear down on loss of signal.

In general, it is not expected that the mobile-to-cell link will notemploy orthogonal Walsh coding in the manner described above. However,in particular applications it may be advantageous to use the samemodulation scheme on the cell-to-mobile and mobile-to-cell links. Underthese circumstances each mobile would utilize the pair of 32768 lengthsector codes as outer codes. The inner code would utilize a Walshsequence, of the length chosen for voice channels, that is assigned tothe mobile for use while it is in that sector. Nominally, the same Walshsequence would be assigned to the mobile for the mobile-to-cell link asis used for the cell-to-mobile link.

In an alternative approach a rate r=1/2, constraint length K=9convolutional code could be used with differential binary phase shiftkeying modulation of the encoded binary symbols. The demodulator in thecell-site could build up a phase reference over a short interval usingthe technique described in the article "Nonlinear Estimation ofPSK-Modulated Carrier with Application to Burst Digital Transmission,"Andrew J. Viterbi and Audrey M. Viterbi, IEEE Transactions OnInformation Theory, Vol IT-29, No. 4, July, 1983. For example, a phasereference could be averaged over only 4 symbols requiring no morechannel coherence than in the above-described scheme.

FIG. 7 provides a block diagrammatic representation of an analogreceiver 400, and digital data receiver 440, included within anexemplary mobile unit CDMA telephone set. Signals directed to receiver400 from a mobile unit antenna (not shown) are provided to adownconverter 500 comprised of RF amplifier 502 and mixer 504. Thereceived signals are provided as an input to RF amplifier 502 where theyare amplified and output as an input to mixer 504. Mixer 504 is providedwith another input, that being the signal output from frequencysynthesizer 506. The amplified RF signals are translated in mixer 504 toan IF frequency by mixing with the frequency synthesizer output signal.

The IF signals are output from mixer 504 to bandpass filter (BPF) 508,typically a Surface Acoustic Wave (SAW) filter having a passband ofapproximately 1.25 MHz, where they are from bandpass filtered. Thecharacteristics of the SAW filter are chosen to match the waveform ofthe signal transmitted by the cell-site. The cell-site transmittedsignal is a direct sequence spread spectrum signal that is modulated bya PN sequence clocked at a predetermined rate, which, as noted above, is1.2288 MHz in the exemplary embodiment.

The filtered signals are output from BPF 508 as an input to a variablegain IF amplifier 510 where the signals are again amplified. Theamplified IF signals are output from IF amplifier 510 to analog todigital (A/D) converter 512 where the signals are digitized. Theconversion of the IF signal to a digital signal occurs at a 9.8304 MHzclock rate in the exemplary embodiment, which is exactly eight times thePN chip rate. The digitized IF signals are output from (A/D) converter512 to the data receiver 440.

Further details of data receiver 440 are depicted in FIG. 7. Datareceiver 440 includes PN generators 516 and 518 which generate thePN_(I) and PN_(Q) sequences in a manner corresponding to that used togenerate the PN sequences at the cell-site. Timing and sequence controlsignals are provided to PN generators 516 and 518 from a mobile unitcontrol processor (not shown). Data receiver 440 also includes Walshgenerator 520 which provides the appropriate Walsh function forcommunication with the mobile unit by the cell-site. Walsh generator 520generates, in response to timing signals (not shown), a data rate selectsignal, and a function select signal from the mobile unit controlprocessor corresponding to an assigned Walsh sequence. The functionselect, and data rate select signals are transmitted to the mobile unitby the cell-site as part of the call set up message. The PN_(I) andPN_(Q) sequences output from PN generators 516 and 518 are respectivelyinput to exclusive-OR gates 522 and 524. Walsh generator 520 providesits output to both of exclusive-OR gates 522 and 524 where the signalsare exclusive-OR'ed and output as the sequences PN_(I) ' and PN_(Q) '.

The sequences PN_(I) ' and PN_(Q) ' are provided to receiver 440 wherethey are input to PN QPSK correlator 526. PN correlator 526 may beconstructed in a manner similar to the PN correlator of the cell-sitedigital receivers. PN correlator 526 correlates the received I and Qchannel data with the PN_(I) ' and PN_(Q) ' sequences and providescorrelated I and Q channel data output to corresponding accumulators 528and 530. Accumulators 528 and 530 accumulate the input information overa period of one symbol, where the number of chips per symbol isequivalent to the Walsh sequence length corresponding to the selecteddata rate. This information is provided to the accumulators 528 and 530via the data rate select signal from the control processor. Theaccumulator outputs are provided to phase rotator 532, which alsoreceives a pilot phase signal from the mobile unit control processor.The phase of the received symbol data is rotated in accordance with thephase of the pilot signal as determined by the control processor. Theoutput from phase detector 532 is the I channel data which is providedto the deinterleaver and decoder circuitry.

Data receiver 440 also includes PN generator 534 which generates theuser PN sequence in response to an input mobile unit address or user IDfrom the mobile unit control processor. The PN sequence output from PNgenerator 534 is provided to diversity combiner and decoder circuitry.Since the cell-to-mobile signal is scrambled with the mobile useraddress PN sequence, the output from PN generator 534 is used indescrambling the cell-site transmitted signal intended for this mobileuser. PN generator 534 specifically provides the output PN sequence tothe deinterleaver and decoder circuitry where it is used to descramblethe scrambled user data. Although scrambling is discussed with referenceto a PN sequence, it is envisioned that other scrambling techniques,including those well known in the art, may also be utilized.

The symbol outputs of the phase detector 532 and the PN generator 534are provided to diversity combiner and decoder circuitry (not shown).The diversity combiner circuitry adjusts the timing of the two streamsof received symbols so as to bring them into alignment, and then addsthem together. This addition process may be proceeded by multiplying thetwo streams by a number corresponding to the relative signal strengthsof the two streams. This operation can be considered a maximal ratiodiversity combiner. The resulting combined signal stream is then decodedusing a forward error detection (FEC) decoder.

Each mobile unit will also generally include baseband circuitry forprocessing the signals generated by the diversity combiner and decoder.Exemplary baseband circuitry may include a digital vocoder of a variablerate type as disclosed in the previously mentioned copending patentapplication. Such baseband circuitry further serves as an interface witha handset or any other type of peripheral device. In this connectionexemplary baseband circuitry is designed to provide output informationsignals to the user in accordance with the information provided from thediversity combiner and decoder.

It is noted that the cell-to-mobile and mobile-to-cell links need not becapable of accommodating an identical set of data rates. In particular,mobile terminals operative to transmit information at a subset of thecell-to-mobile data rates are not required to be designed to generateWalsh codes at other rates. It follows that existing mobile units may beincorporated into a variable data rate CDMA cellular system having acell-to-mobile link implemented in accordance with the invention.

Although in the exemplary embodiment described above a single Walsh codesequence remains assigned to a given voice channel for the duration ofthe mobile telephone call, the present invention is not limited to suchan implementation. In particular, under certain conditions it isconceivable that efficiency could be enhanced by changing the length ofthe assigned Walsh sequence while a call is in progress. This type ofscheme would likely be effected by monitoring voice activity in a mannerfacilitating periodic reassignment of, for example, codes of relativelyshorter length to channels characterized by higher levels of voiceactivity or higher rates of data transmission.

The previous description of the preferred embodiments is provided toenable any person skilled in the art to make or use the presentinvention. The various modifications to these embodiments will bereadily apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other embodiments without the use ofinventive faculty. Thus, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features disclosedherein.

I claim:
 1. A modulation system for use in a spread spectrumcommunications system, said modulation system being disposed to transmitan information signal, comprising:means for selecting a first functionfrom a set of functions in accordance with a selected characteristic ofsaid information signal, said set of functions including a plurality ofsubsets of functions, wherein a predefined recursive relationship existsamong the functions within each of said subsets, said means forselecting including means for preventing selection of more than a singlefunction from each of said subsets, and members of said subsets areorthogonal to members of all other subsets; means for generating afunction signal representative of said first function; means forgenerating a pseudorandom noise (PN) signal of a predetermined PN code;and means for combining said function signal, said PN signal and saidinformation signal, and for providing a resultant first modulationsignal.
 2. The system of claim 1 wherein said set of functions aregenerated from the Walsh functions of varying length means for selectingan orthogonal function includes means for deriving said set of functionsby generating a set of Walsh functions of variable length.
 3. The systemof claim 2 wherein said means for selecting further including means foridentifying as available for selection a subset of said set of Walshfunctions on the basis of said data rate.
 4. The system of claim 2wherein said means for generating said set of orthogonal Walsh functionsincludes means for recursively deriving a set of orthogonal Walshfunctions of varying length from a root function.
 5. The system of claim4 further including controller means for preventing subsequent selectionof ones of said Walsh functions within said subset of Walsh functionsupon selection of one of said Walsh functions within said subset ofWalsh functions.
 6. The system of claim 5 wherein said controller meansincludes means for preventing selection of said root Walsh sequence whensaid selected orthogonal function consists of one of said Walshsequences within said set of Walsh sequences.
 7. The system of claim 4wherein selected ones of said Walsh sequences are of a length equivalentto a maximum power of two, and remaining ones of said Walsh sequencesare of lengths equivalent to predetermined powers of two less than saidmaximum power of two.
 8. The system of claim 2 wherein said predefinedcharacteristic corresponds to a data rate of said information signal. 9.The system of claim 1 further comprising encoder means for receiving anderror correction encoding said information signal, and for providing anerror correction encoded information signal to said means for combiningfor combination with said orthogonal function signal.
 10. The system ofclaim 1 further including means for generating a set of orthogonal Walshfunctions comprising said set of orthogonal functions, said setincluding subsets of said orthogonal Walsh functions derived from rootfunctions of said subsets; andcontroller means for tracking which ofsaid root functions are being used in said communication system, and fordesignating as unavailable those functions included within the subsetsassociated with said root functions being used.
 11. The system of claim1 further including means for preserving for selection at least a firstof said orthogonal functions included in a first of said subsets. 12.The system of claim 1 further including means for deselecting at leastone previously selected orthogonal function within a given one of saidsubsets in order to make available for selection another orthogonalfunction in said given subset.
 13. A spread spectrum modulator forprocessing first and at least a second digital user information signalsin preparation for respective transmission to first and second intendedrecipient users, comprising:means for selecting first and secondfunctions from a plurality of functions in accordance with first andsecond data rates of said first and second information signals,respectively, said plurality of functions being divided into a pluralityof subsets wherein a redefined recursive relationship exists among thefunctions within each of said subsets, said means for selectingincluding means for preventing selection of more than a single functionfrom each of said subsets; function generator means for generating firstand second Walsh function signals corresponding to said first and secondfunctions; first combiner means for providing a first intermediatemodulation signal by combining said first information signal and saidfirst Walsh function signal, and for providing a second intermediatemodulation signal by combining said second information signal and saidsecond Walsh function signal; pseudorandom noise (PN) generator meansfor generating a PN signal of a predetermined code function; secondcombiner means for providing a first output modulation signal bycombining said PN signal and said first intermediate signal, and forproviding a second output modulation signal by combining said PN signaland said second intermediate signal.
 14. The modulator of claim 13wherein said first and second digital user information signals arecomprised of frames of variable rate vocoded voice data.
 15. A signalmodulation method for for use in a spread spectrum communication systemwithin which is processed at least a first information signal,comprising the steps of:selecting an function from a plurality offunctions in accordance with a predefined characteristic of saidinformation signal, said set of functions including a plurality ofsubsets of functions wherein a predefined relationship exists among thefunctions within each of said subsets, said step of selecting includingthe step of preventing selection of more than a single function fromeach of said subsets; generating an function signal representative ofsaid selected function; generating a pseudorandom noise (PN) signal of apredetermined PN code; combining said function signal, said PN signaland said first information signal, and providing a resultant firstmodulation signal.
 16. The method of claim 15 further including the stepof generating said plurality of orthogonal functions by generating a setof orthogonal Walsh functions in which a subset of said set oforthogonal Walsh functions is derived from a root one of said Walshfunctions.
 17. The method of claim 16 further including the step ofpreventing selection of ones of said Walsh functions within said set ofWalsh functions upon selection of one of said Walsh functions withinsaid set of Walsh functions.
 18. The method of claim 15 furtherincluding the step of preserving for selection at least a first of saidorthogonal functions included in a first of said subsets.
 19. The methodof claim 15 further including the step of deselecting at least onepreviously selected orthogonal function within a given one of saidsubsets in order to make available for selection another orthogonalfunction in said given subset.
 20. A method for processing first andsecond digital user information signals for respective transmission toat least first and second intended recipient users, comprising the stepsof:selecting first and second orthogonal functions from a plurality oforthogonal functions in accordance with first and second data rates ofsaid first and second information signals, respectively, said orthogonalfunctions being divided into a plurality of subsets wherein a predefinedrecursive relationship exists among the orthogonal functions within eachof said subsets, said means for selecting including means for preventingselection of more than a single orthogonal function from each of saidsubsets; generating first and second Walsh function signalscorresponding to said first and second orthogonal functions; providing afirst intermediate modulation signal by combining said first informationsignal and said first Walsh function signal, and providing a secondintermediate modulation signal by combining said second informationsignal and said second Walsh function signal; generating a pseudorandomnoise (PN) signal of a predetermined code function; providing a firstoutput modulation signal by combining said PN signal and said firstintermediate signal, and providing a second output modulation signal bycombining said PN signal and said second intermediate signal.
 21. Themethod of claim 20 further including the steps of:generating first andsecond sets of Walsh function signals of varying length from said firstand second Walsh function signals, respectively; designating asunavailable said first Walsh signal upon selection of one of said Walshfunction signals included within said first set, and designating asunavailable said second Walsh function signal upon selection of one ofsaid Walsh function signals included within said second set; andassigning to users ones of said Walsh function signals so as to minimizethe number of Walsh function signals designated as unavailable.
 22. Amodulation system for use in a spread spectrum communication system,said modulation system being disposed to receive an information signal,comprising:means for selecting an orthogonal function from a set offunctions in accordance with a selected characteristic of saidinformation signal, said set of functions including a plurality oforthogonal functions divided into a plurality of subsets wherein apredefined recursive relationship exists among the orthogonal functionswithin each of said subsets, said means for selecting including meansfor preventing selection of more than a single orthogonal function fromeach of said subsets; means for generating an orthogonal function signalrepresentative of said selected orthogonal function; and means forcombining said orthogonal function signal and said information signal,and for providing a resultant first modulation signal.
 23. The system ofclaim 22 wherein said means for selecting an orthogonal functionincludes means for deriving said set of functions by generating a set ofWalsh functions of variable length.
 24. The system of claim 23 whereinsaid predefined characteristic corresponds to a data rate of saidinformation signal, said means for selecting further including means foridentifying as available for selection a subset of said set of Walshfunctions on the basis of said data rate.
 25. The system of claim 22further including means for deselecting at least one previously selectedorthogonal function within a given one of said subsets in order to makeavailable for selection another orthogonal function in said givensubset.